Both products are quite impressive in specs. However, Guitar Rig 2 has more flexibility and routing options. You can plug anything into everything. In Amplitude 2, you limited by the 2 chains in this particular order
(Tuner -> Stomp Effect -> Amp -> Cab/Mic -> Rack Effect)
Still, you're able to configure parallel or serial effects, juz route them accordingly.
Soundwise, after listening to both product demos, I personally prefer the Amplitude's sound quality even though I haven't heard all those classic amps in real life.
Also in Amplitude, you can "built your own amp" with pre-amp, EQ and power amp modules.
Here's a short summary of both programs
Guitar Rig 2
Pros:
-Unlimited routing options. Can plug anything into everything.
-Friendly GUI. Simple to understand and easy to use.
-Lots of effect modules.
Cons:
-Average sound quality.
Amplitude 2
Pros:
-Able to "built custom amps".
-Nice graphics and friendly GUI.
-Excellent sound quality.
Cons:
-Limited routing options.
-Limited effect modules.
Well, nothing's perfect. Each has its own strength and weakness. It's best to try them out before deciding which one suits you.